how to add frame numbers over footage in blender video editor
In the scene preferences turn ON metadata. Select the information that you would like to see in the frame. You can choose to see the scene properties but not the strip properties unfortunately*.



Here is the overlay in the render view, only available when a new frame is rendered. 



Sorry it does not seem to be available in the VSE live view. You could keep a little UV Image editor window open and hit f12 occasionally.

*According to this code entry for the Metadata function https://developer.blender.org/D1592 you cannot access Sequence Strip information.